In recent years, there is a trend toward the use of water-based coating compositions, particularly aqueous emulsion-based coating compositions, since organic solvent-containing coating compositions are undesirable in view of effective utilization of resources.
However, in general aqueous emulsion-based coating compositions, at a drying time, water is dissipated from an emulsion-polymerized resin dispersed in water, and the particles of the resin are mutually fused to each other to form a coating, so that the formed coating is poor in denseness and is therefore poor in durability against water as compared with organic solvent-containing coating compositions.
For overcoming the above defect, there is known a method in which the a resin obtained by emulsion-polymerization is extremely decreased in diameter to improve the denseness of a coating. Even if a dense coating is formed, however, it is impossible to attain the durability against water, weathering resistance and pollution resistance equivalent to those attained by a silicon-containing polymer.
As a method of improving a coating in durability against water, weathering resistance and water repellency, there is known a method in which a resin is synthesized by copolymerizing dimethylpolysiloxane having a radical-polymerizable double bond. However, when the resin is synthesized in an emulsion polymerization system, various problems are caused. For example, a dimethylpolysiloxane having a radical-polymerizable double bond shows poor reactivity with an ethylenically unsaturated monomer since the dimethylpolysiloxane has a high molecular weight, as high as 1,000 to 10,000, and as a result, at an emulsion polymerization time, aggregates are formed, or gelation or separation from water takes place. There is therefore a defect that it is difficult to obtain a water-based dispersion stable with the passage of time.
Further, a resin dispersion obtained by the above method is an emulsion having large particle diameters in most cases. Therefore, even if a dimethylpolysiloxane excellent in durability against water, weathering resistance and water repellency is used for modification, the defect is that no effect produced by the modification with silicone can be obtained due to a coating defect caused by the fact that the resin has a large particle diameter.
It is an object of the present invention to provide a water-based resin dispersion which is modified with dimethylpolysiloxane and is stable with the passage of time, whose resin has a very fine particle diameter and which has excellent durability against water.